This invention relates to internal combustion engines of the type comprising twin cylinders arranged in a vee in a plane normal to the crankshaft. More particularly, the invention relates to the camshaft included in such engines, which camshaft actuates the valves that control the flow of air/fuel mixture into and exhaust gas out of the cylinders.
A common type of motorcycle engine is the so-called xe2x80x9cvee-twinxe2x80x9d engine in which the two cylinders are arranged in a vee with the cylinders lying on a plane which is transverse to the crankshaft and normal thereto. The axes of the cylinders meet at the axis of the crankshaft.
Such engines also include a camshaft which is typically in the crankcase where it is driven by a pinion gear of the crankshaft. The camshaft, via pushrods and rocker arms, actuates valves which control the influx of air/fuel mixture from the carburetor and the efflux of combusted mixture.
Vee-twin engines are also known that have twin camshafts. Such engines usually have superior performance over an otherwise identical engine because of the improved pushrod geometry which gives better operation of the valves. The twin camshafts are driven by a chain running from a gear on the crankshaft.
The chain drive of the twin camshafts of engines of the foregoing type is unsatisfactory because of a limited service life due to the chain requiring adjustment or even replacement. Furthermore, chain breakage can occur with damage to the engine and in some instances injury to the rider of a motorcycle powered by the engine.
It would therefore be desirable to have available an internal combustion engine of the vee-twin configuration with dual camshafts wherein the camshaft drive mechanism is not a chain drive.
The object of the invention is to provide a camshaft drive mechanism for an internal combustion engine of the vee-twin configuration which does not use chain drive.
In a broad format, the invention provides a camshaft drive mechanism for an internal combustion engine of the type comprising twin cylinders arranged in a vee in a plane normal to the axis of the crankshaft of said engine, wherein said mechanism comprises a first camshaft for actuating inlet and exhaust valves of one of said cylinders and a second camshaft for actuating inlet and exhaust valves of the other of said cylinders, which camshafts rotate on axes parallel to the axis of said crankshaft, and wherein said camshafts are driven by a gear on said crankshaft which engages a primary gear on said first camshaft with an adjacent secondary gear on said first camshaft imparting counter-rotation on said second camshaft via an identical secondary gear on said second camshaft.
The inventors have found that by replacing the chain-drive of twin camshaft vee-twin engines with a gear-drive, the disadvantages referred to above of a chain drive are overcome. Furthermore, the inventors have surprisingly found that the long-term performance of an engine including the camshaft drive mechanism of the invention does not deteriorate, as does the performance of an engine with chain-driven camshafts.
A particular problem faced by the inventors in configuring a gear-drive mechanism for twin camshafts was to minimize the space occupied by the gears yet maintain lateral alignment of gears. This was achieved by use of thrust interfaces as will be explained in greater detail below.
The camshaft drive mechanism of the invention can be used for vee-twin engines having a vee of any angle. The mechanism is particularly suited for vee-twin motorcycle engines having a vee of 45xc2x0 to 60xc2x0. The camshaft axis is advantageously in alignment with the axis of the cylinder with which it is associated. However, the camshaft axis can be offset by as much as 10xc2x0 from the cylinder axis.
The crankshaft drive gear and primary camshaft gearxe2x80x94which in effect is an idler gearxe2x80x94are configured such that a camshaft turns at half the speed of the crankshaft. The diameters of the secondary gears are determined by the distance apart of the camshafts although it will be appreciated by one of ordinary skill in the art that the diameter of a secondary gear must be greater than the radius of the primary gear so that the secondary camshaft clears the primary gear.
The camshafts per se can be of any type or configuration suitable for use in vee-twin engines. Camshafts can have multiple lobes for actuatingxe2x80x94via pushrods and rocker armsxe2x80x94more than one inlet and/or exhaust valve per cylinder.
A further embodiment of the invention is a crankcase incorporating the camshaft drive mechanism of the invention. This crankcase includes, as part of the casting, tappet blocks which house the cam followers. Conventional engines employ separate tappet blocks which are fixed to the crankcase. An integral tappet block contributes to the strength of the crankcase in the area of the cam chest.
One of ordinary skill in the art will appreciate that the more rigid drive train of the camshaft drive mechanism of the invention reduces vibration.
The invention also includes within its scope an engine comprising the camshaft drive mechanism described above.